The use of optical communication promises to significantly enhance communications and computing power. This is because more information can be encoded when transmitting light than when transmitting electrical signals. In order to represent (or encode data) and transmit data with light, the transmitted light signal must be controlled so that the receiver can detect the information being sent. Controlling a light signal means transmitting a light signal with a very well defined wavelength and being able to modulate the transmitted light wavelength. Quantum well semiconductor lasers are very useful for optical communications because they are small and emit light which has a well defined wavelength. Therefore, the laser can be mounted next to a small optical fiber transmission guide and directly emit light having a defined wavelength into the guide.
The problem with semiconductor lasers in optical communications is that the emitted radiation frequency cannot be quickly tuned or significantly modulated. This is important in optical communications because the amount of data which can be transmitted over a single guide dramatically increases when the data can be encoded using different frequencies of light. The frequency of light emitted from semiconductor lasers depends upon the relative positions of the conduction and valence band edges in the materials which form the laser. Changing the relative positions of the conduction and valence bands typically requires changing the temperature of the device or exerting a mechanical pressure on the lattice structure of the device. Applying these types of physical conditions to the laser, in order to tune the frequency of light output, is much slower than is required in order to encode data for optical transmission. Therefore, semiconductor lasers typically cannot encode data using the frequency of the emitted light and, as a result, the capacity for data transmission with a semiconductor laser over a transmission guide is significantly decreased.
Generally, light is emitted from a semiconductor structure due to an electron making a transition from the conduction band of the semiconductor material to the valence band and losing energy in the process. The lost energy is equal to the difference in energy between the conduction band and valence band edges plus the energy above and below the band edges for any one particular electron transition. The energy lost by an electron when making the transition from the conduction band to the valence band is emitted from the semiconductor as light. The light has a frequency which is proportional to the lost energy and a wavelength which is inversely proportional to the lost energy. Since electrons making the transition from the conduction band to the valence band generally have many different values of lost energy, the emitted light does not have a single wavelength or even a narrow band of wavelengths. Typically, the radiation emitted from a semiconductor has a broad spectrum of wavelengths.
In contrast to typical semiconductor light emitting devices, the semiconductor laser emits light having a narrow band of wavelengths. This is because many electrons make transitions between well defined energy bands. The energy bands are well defined because lasing at one wavelength tends to suppress the lasing at other wavelengths. The laser output is useful but the use is limited because the range of output wavelengths is limited. The semiconductor laser has a limited range of output wavelengths because the wavelength is controlled by the material which makes up the laser. One way in which to modify the output wavelength of a laser fabricated with a particular material is to make a quantum well laser. A quantum well laser has multiple discrete conduction and valence states whose energies depend on the width of the quantum well. The quantum well laser output light has a narrow band of wavelengths which is different than the wavelengths produced from a typical semiconductor laser made from the same material and the difference depends on the quantum well width. In effect, the output of a typical semiconductor laser made from a particular material can be changed by forming a quantum well laser from the same material.
The problem with both the typical semiconductor laser and the quantum well laser is that they are not capable of tuning the wavelength of the emitted light once the laser is fabricated. The prior art has attempted to solve this problem by changing the physical conditions of the quantum well laser. These types of physical changes are too slow for practical use of the quantum well laser in communication applications as noted above. The prior art has also attempted to use an applied electric field to modulate both the wavelength and the intensity of a quantum well laser. The proposed devices would operate by modulating the optical transition energies of a quantum well and by changing the number of electrons and holes which recombine as a function of position in a quantum well. The problem with the intensity modulating devices is that the devices merely turned the laser on or off. Lasing only occurred in these devices when there was no electric field present and the devices used the electric field to turn off the laser. The problem with single quantum well lasers which attempt wavelength modulation is that the electric field required to shift the wavelength of the laser is too large for the small shift which is produced. This is because the wavelength shift in a single quantum well is proportional to the square of the electric field and the fourth power of the width of the quantum well. Therefore, large changes in the electric field are required to produce small changes in the wavelength.